Elimination of floating slime during electrolytic refining of copper

ABSTRACT

A method for eliminating floating slime in the electrolytic refinement of copper. The method uses a novel electrolyte which contains specific quantities of trivalent arsenic, pentavalent arsenic and pentavalent antimony; the arsenic contents being obtained and maintained by direct addition of the substances to the solution, or by increasing the arsenic content in the anode.

United States Patent [1 1 Lindstrom I ELIMINATION OF FLOATING SLIMEDURING ELECTROLYTIC REFINING OF COPPER Inventor: Nils Folke RuneLindstrom,

Skelleftehamn,-Sweden Assignee: Bollden Aktiebolag, Stockholm,

Sweden Filed: May 25, 1971 Appl. No.: 146,850

Foreign Application Priority Data May 28, 1970 Sweden 7380/70 US. Cl.204/108, 204/293 Int. Cl C22d 1/16, BOlk 3/06 Field of Search 204/106,108, 293

11] 3,755,111 [451 Aug.-28, 1973 References Cited 7 A PrimaryExaminer-John H. Mack Assistant Examiner-R. L.- Andrews An0rneyStevens,Davis, Miller and Mosher [57} ABSTRACT A method for eliminating floatingslime in the electrolytic refinement of copper. The method uses a novelelectrolyte which contains specific quantities of trivalent arsenic,pentavalent arsenic and pentavalent antimony; the arsenic contents beingobtained and maintained by direct addition of the substances to thesolution, or by increasing the arsenic content in the anode.

6 Claims, No Drawings 1 ELIMINATION OF FLOATING SLIME DURINGELECTROLYTIC REFINING OF COPPER The present invention relates to amethod for eliminating floating slime when refining copperelectrolytically with copper anodes which are contaminated with antimonyand/or bismuth.

Copper refining by electrolysis is effected between an anode and acathode in an electrolyte comprising an aqueous solution of coppersulphate, the copper content normally being 35-50 g/l, and inthevpresence of approximately 150-250 g/l sulphuric acid, whichincreases the electrical conductivity of the electrolyte. It is knownthat arsenic together with antimony and bismuth in solution formsso-called floating slime, which is deleterious to the current efficiencyand the cathode quality.

The electrolysis is normally carried out at temperatures between 55 and65C. At temperatures higher than 65C the amount of water evaporated fromthe electrolyte is extremely high, which ultimately results in higherheating costs. Moreover, at such high temperatures the cathode structureand therewith the quality of the cathode copper is impaired. Attemperatures below 55C there is a risk of anode passivation.

The anode normally contains from 98 to 99.5% copper.

When the anode is dissolved electrolytically, metals. of more noblecharacter than copper, such as silver, gold and the platina metals areinsoluble and together with other insoluble impurities in the copper,such as selenides and tellurides, form a grayish-black to blackslimewhich is initially built up as a 0.5-1 cm thick coating on the anodesurface but which gradually loosens and sinks to the bottom of theelectrolytic cell. The layer of slime on the anode, so called anodeslime, also contains finely divided, copper powder due to the fact thatthe anode contains oxygen. The oxygen in the anode copper is present,iner alia, as copper(l)oxide, Cu O. Copper(l)oxide is dissolved in theelectrolyte according to the reaction Cu O H 80 CuSO H O Cu because atequilibrium Cu Cu 2Cu the reaction is considerably moved to the left.The copper powder formed is incorporated in the anode slime. Copperpowder is also formed in the anode slime owing to the fact that thereaction Cu Cu 2e" is slower than the reaction Cu Cu +e. In this way asurplus of Ca ions is formed adjacent the anode surface, which ions, inaccordance with the above equilibrium, are later disproportioned to Cu-ions and finely divided copper.

The lead present in the system presses primarily into solutions as Pb-ions, but is immediately precipitated as PbSO, and follows the anodeslime.

The tin present in the system primarily passes into solution as Sn-ions,but is then precipitated as tin(lV)- hydroxide in gel form and alsofollows the anode slime.

The nickel and arsenic present in the system pass practically completelyinto solution.

The Ni -ions are not precipitated and the concentration of Ni* -ions inthe electrolyte would increase if the electrolyte were not drained OEand replaced with freshly produced nickel free electrolyte. By removinga quantity of saturated electrolyte and replenishing with freshelectrolyte, the contamination of nickel in the cathode can bemaintained at an acceptably low level, as this contamination normallybeing solely in the form of electrolyte inclusions. The drainedelectrolyte can be evaporated and the nickel sulphate and the coppersulphate can be recovered. Residual copper in the drained electrolyteis. then recovered by electrolysis using insoluble anodes. The presenceof nickel-ions in the solution also reduces the solubility of the coppersulphate and lowers the electrical conductivity of the electrolyte.

The amount of arsenic contained by the anode normally exceeds thequantity required stoichiometrically for precipitating antimony andbismuth, and hence draining off the electrolyte also assists inregulating the arsenic content in the solution. Draining of theelectrolyte also serves to maintain a constant content of copper in theelectrolyte. For example, if no electrolyte were drained off, the coppercontent of the electrolyte would increase as a result of theaforementioned reaction and because oxygen dissolved in the electrolyteoxidizes the finely divided copper powder contained in the anode slimeand the electrodes which consist of copper, whereupon copper(il)ionspass into solution. The fresh electrolyte which is used to replace thedrained electrolyte should therefore be free from nickel and arsenic andhave a low copper content. It is often an advantage to use anelectrolyte which comprises solely diluted sulphuric acid. The coppercontent in the electrolyte can also be kept at a constant level byintroducing insoluble anodes, for instance lead, in one or more cells.Thus, it is possible to reduce the copper content without increasing thedrainage. In this way it is possible to keep a high arsenic content inthe electrolyte without taking further measures. Soluble oxygen in theelectrolyte also oxidizes the three valent arsenic and three valentantimony present to the'five valent state. It is presumed that Cu(l)ionscatalize this oxidation.

Practically all the bismuth present passes into solution in trivalentform and is not oxidized by atmospheric oxygen dissolved in theelectrolyte.

During the electrolytic process a drop in voltage is obtained owing toan ohmic resistance in the busbars, electrical contacts and the ohmicresistance in the electrolyte, together with so called activationpolarisation, caused by the resistance transforming copper ions in theelectrolyte to copper atoms in the metal lattice and vice versa, and socalled concentration polarisation, caused by differences in ionconcentration adjacent the electrodes in relation to the concentrationin the main body of the electrolyte. Movement of the copper ions betweenthe anode and the cathode takes place mainly through the process ofconvection and diffusion, while current transport ismainly effected withhydrogen ions. Thus, a thin layer of electrolyte is formed around thesurfaces of the electrodes having a composition different to the mainbody of the electrolyte. Circulation in a conventional electrolytic cellcannot affect the composition of these films to any appreciable extent.The cathode flm becomes deplete of copper ions and slightly enrichedwith sulphuric acid, and consequently the density becomes lower than thedensity of the main body of the electrolyte, whereby the depletedelectrolyte flows upwards along the cathode. In turn, the anode film isenriched with copper ions and slightly depleted of sulphuric acid andthereby obtains a greater density than that of the main body ofelectrolyte, and hence the anode film enriched with copper sulphateflows downwards along the anode. This behavior of the films causes anenrichment of copper sulphate in the lower parts of the electrolyticcell. To avoid an uneven result, this must be counteracted by some formof vertical circulation of the electrolyte through the cell. A verticalcirculation can be achieved by removing the electrolyte either from theupper part of the cell and admitting electrolyte to the lower part orvice versa, at the same time as a certain horizontal circulation iseffected by admitting the electrolyte to one end of the cell andremoving it from the other. The differences in the concentration of thecopper sulphate between the upper part and the lower part of the cell ismore efl'ectively equilized with increasing circulation rate. On theother hand, at high circulation rates there is a risk of the anode slimeswirling. A normal circulation rate is -20 l/min through an electrolysistank containing 5,000 l.

The amount of energy consumed per unit of precipitated copper increaseswith increasing current density. The fact that with an increased currentdensity it is possible to increase the production rate and obtain a morerapid run of material is naturally an economic advantage. In general,the consumption of energy has small economical significance incomparison with the capital costs, and therefore most plants operatewith the highest possible current density, normally 200-270 A/m.

In order to obtain a high quality of the cathode copper, it is necessaryto use different organic additives. Glue, thiocarbamides, goulac arenormally used in this respect.

With technical electrolysis the size of the cathode and anode isnormally selected on the basis of a compromise made between differenteconomical and technical view points, and is generally approximately 1 Xl m. In certain instances the electrolysis process is carried out witheach electrode in series, whereby one side of the electrode serves as acathode and the other side as an anode. As a rule, however, all cathodesand anodes of each cell are connected in parallel. The distance betweenthe centre lines of the anodes is normally 9-13 cm and the cathodes areplaced centrally therebetween.

With increasing distance between the electrodes the risk of fallingslime from the anodes contaminating the cathodes decreases and, at thesame time, a better cathode structure is obtained. On the other hand,plant costs increase with increased electrode spacing.

When producing electrolytic copper, a floating slime is normally formedwhich consists of an oxide complex of bismuth, arsenic and antimony inproportions which are dependent on the concentration of these substancesin the electrolyte. This concentration depends primarily on the contentsof said elements in the anodes. Floating slime shows very small tendencyto settle, remains suspended in the electrolyte and impurities thecathode copper and causes growths to form on the cathode, which canresult in short circuiting between the anode and the cathode and therebyreduce the current effeciency. Moreover, growths on the cathode act assettling surfaces for the anode slime.

Despite the fact that a large number of works have been published inwhich attempts have been made to establish the reasons why floatingslime is formed, no one has hitherto been able to find the causalconnection. Among others, Livshits and Pazukin (J. Applied Chemistry ofthe USSR, 27 (1954), page 283-292) have carried out a comprehensiveexamination and reached the conclusion that the only way in whichfloating slime can be avoided is to reduce the antimony content in theanodes to an extend such that the content of antimony in the electrolytelies beneath 0.5 g/l. G. Graf and A. Lange also discuss in Neue Hutte 10(I965 page 2 1 6220, the reasons for the formation of floating slime andreport that this depends on the formation of antimony arsenate inamorphous form. Graf and Lange also recommend a restricted amount ofantimony in the anode. None of these publications indicate or show amethod for producing high grade cathode copper from anodes with highcontent of antimony.

It has been assumed in the aforementioned publications that the floatingslime derives from the precipitation of trivalent antimony withpentavalent arsenic, the latter being oxidized by oxygen which isdissolved in the electrolyte. According to Graf and his colleagues,trivalent antimony is oxidized to pentavalent antimony and withsufficiently high contents is also precipitated as Sb,O

The oxidation of antimony (III) and arsenic (III) to pentavalent ions inthe absence of a catalyst is extremely slow, but, as previouslymentioned, the presence of monovalent copper is considered to catalysethe oxidation by forming 0, according to the reaction Cu+ 0 9 Cu 0;.

When the antimony content of the anode is greater than approximately 400g/t and/or the bismuth content of the anode is greater thanapproximately 200 g/t it has not previously been possible to effectuatea copper electrolysis obtaining a high grade cathode copper. As thequality and purity demands placed on the cathode copper is continuallyincreasing the contaminants which deleteriously affect the qualityproperties of the copper must be maintained as low as possible. Antimonyand/or bismuth strongly impair the quality of cathode copper. Normally,the quality of the cathode is valued by deterimining the soft annealingtemperature (recrystallisation temperature). Even relatively smallcontents of antimony and/or bismuth will result in an excessively highsoft annealing temperature,

which renders the product less suitable for important fields of use,particularly such fields as the manufacture of fine wire filaments etc.It is true that the major content of the antimony can be removed duringthe processing of the copper raw materials to anode copper but it isstill very difficult and expensive to attain the desired low antimonycontent. It is still more difficult to remove bismuth during thesetreatment stages. Consequently, it has hitherto been impossible toutilize raw copper material with high antimony and/or bismuth contentfor producing high grade electrolytic copper.

DESCRIPTION OF THE INVENTION The present invention is concerned with amethod for electrolytically refining copper anodes which contain morethan 200 grams of antimony per ton and/or more than grams of bismuth perton without getting floating slime, and is characterized by the steps ofregulating the quantities of trivalent arsenic, pentavalent arsenic andpentavalent antimony in the electrolyte so that in the steady state thecontent of As(II) exceeds 1 g/l, the content of As(V) exceeds 2 g/l andthe quantity of Sb(V) is less than 0.5 g/l.

Thus, in accordance with the invention trivalent arsenic can beintroduced to the electrolyte in quantities at which the content oftrivalent arsenic at the steady state is maintained above 1 g/l,preferably between 2 and 5 g/l. The trivalent arsenic present in thesystem will absorb oxygen dissolved in the electrolyte, the arsenicbeing, at the same time oxidized to pentavalent arsenic. At the sametime, the electrolyte shall contain at the steady state at least 2 g ofAs(V)/l preferably 7-l5 g/l, causing crystalline arsenates of trivalentantimony and bismuth to precipitate. in the case of anodes which do notcontain antimony but which do contain bismuth, the content ofpentavalent arsenic is of greater significance than the content oftrivalent arse- The increase of As(lll )-content of the electrolyte canbe caused by dissolving a suitable As(lll) compound, for example AS203,in the electrolyte or by adding an aqueous solution of As 0 thereto. Itis also possible to admit arsenic to the electrolyte via the anodes,from which it passes into solution as trivalent arsenic.

The present invention thus includes a method for completely avoiding theformation of flotation slime in the case of anodes which contain highcontents of antimony, and it has been discovered that anodes having, forexample, up to 2,500 grams of antimony per ton can be used successfully.Furthennore, the present invention solves the problems which occur whenthe anodes contain large quantities of bismuth, for example, up to 2,500g/t. Theoretically, anodes which have higher contents of antimony andparticularly bismuth can be used since in respect of the formation offloating slime no upper limit with regard to the antimony and bismuthcontents has been observed.

The mechanism by which floating slime is formed has now beenestablished, and above all the key role played by pentavalent antimony.It has namely been discovered that antimony and arsenic pass intosolution in trivalent form and that further oxidation to pentavalentform is primarily catalysed by elementary copper. In the conventionalelectrolysis of copper which contains antimony a certain amount ofpentavalent antimony is always formed as a result of oxidation of theantimony with atmospheric oxygen which to some extent is dissolved inthe electrolyte. This amount is normally of the order of 0.05-0.20 g/l,while in the presence of antimony(V) non-settling amorphous floatingslime precipitates are obtained. These precipitates are chemicallynon-defined compounds existing between pentavalent antimony, trivalentantimony, trivalent bismuth (when present), pentavalent arsenic, oxygenand water. Trivalent arsenic is not included in the floating slime.Floating slime gives no defraction lines when subjected to X-raydefraction analysis. The composition of the compounds defining floatingslime varies with the concentration of the components in theelectrolyte, although the atomic relationship between arsenic and thesum of antimony and bismuth and normally lies between 1:15 and 1:2. Itis probable that the presence of elementary copper is necessary for theoxidation of arsenic(IlI) and antimony(lll), since peroxides are formedon the surface of the copper, which in turn oxidize arsenic(lll) andantimony(lll). It has been discovered that the rate of oxidation ishigher with the oxidation of arsenic(IIl) than with the oxidation ofantimony(lll), and hence arsenic(lll) primarily absorbs the oxygendissolved in the electrolyte. It has been established that pentavalentantimony is deleterious to the copper electrolysis not only because itcontributes to the formation of floating slime but also because itretards the precipitation of antimony and/or bismuth arsenates. Aconventional electrolyte thus becomes highly oversaturated with respectto bismuth and antimony.

It has also established that if no pentavalent antimony is present or ifonly present in quantities below approximately 0.05 g/l electrolyte,only crystalline precipitates of antimony arsenate and/or bismutharsenate are formed and no floating slime. X-ray defraction analysis ofthe precipitates clearly shows lines of well-formed crystalline phases.The atomic relationship between arsenic and the sum of antimony andbismuth is always equal to one. Antimony arsenate is a defined chemicalcompound which has been found to have monoclinic crystal modificationwith the edge length of a=5.3l8 Angstrom, M917 Angstrom and c==4.8l4Angstrom and the monoclinic angle 3 9375. The composition is probablySbOH,AsO Similarly, bismuth arsenate is a defined compound which occursin two crystal modifications, tetragonal (the stable) of monoclinic,with the composition BiAsO The crystalline precipitates settle readilyand behave principally in the same manner as, for example, PbSO and sinkto the bottom of the tank together with other anode slime.

The method of the present invention thus prevents the formation ofpentavalent antimony to an extent such that the concentration in thesolution is maintained below 0.05 g/l, preferably below 0.02 g/l. Inthis way, formation of the previously described floating slime isrendered impossible and crystalline antimony arsenate and bismutharsenate percipitate onto graft crystals present in the anode slime. Ifminor quantities of pentavalent antimony are formed these are removed byco-crystallisation. The crystallisation also causes the total contentsof antimony and bismuth in the electrolyte to be greatly reduced.

In the addition to the use of trivalent arsenic, as a mean of preventingthe formation of pentavalent antimony, oxidation of trivalent antimonyby ambient air can be prevented by excluding contact of the electrolyteand atmospheric air by means of air tight pumps and an air tightcirculation system, for example. The content of pentavalent antimony canbe maintained in this way below 0.05 g/l. It is also possible to reducethe quantity of oxygen dissolved in the solution by using an inert gasto protect the circulation system. The electrolytic cells can beprotected against atmospheric oxygen by known expedients devisedv forthe purpose of reducing evaporation of the electrolyte surface, forexample expedients as covering the electrolyte with buoyant plasticbodies, plastic cloths or an oil layer.

EXAMPLE The experiment was carried out on factory scale and theelectrolyte partially protected against oxidation by ambient air byusing an air tight circulation system. As- (Ill) was supplied to thesystem as a reduction agent.

A conventional electrolyte comprising 40 g/l Cu, 170 g/l H 4 g/l As, ofwhich 0.5 g/l was As(Hl), 0.43 g/l Sb of which 0.10 g/l Sb(V) and 0.35g/l Bi was used during the initial stages of the experiment. Thisrepresents a conventional electrolyte at the steady state.

, Trivalent arsenic was added in the form of an aqueous solutioncontaining 30 g/l A3 0 (saturated at room temperature). During the firsttwo weeks, an aqueous solution containing 60% As 0 was also added. Theanodes comprised and anode copper having 98% Cu,

0.40% Ni, 040% Ag, 0.11% As, 0.035% Sb and The experiment was conductedin 28 conventional copper electrolytic cells in commercial operation.The steady state was reached after three weeks, the contents of theelectrolyte being approximately 4 g/l As- (Ill) and approximately 11 g/lAs(V). The total anti mony content stabilized at least than 0.02 g/l ofwhich was Sb(V) less than 0.02 g/l, and the bismuth content at less than0.15 g/l. During the running-in time, cathodes of normal quality wereinitially obtained. The quality of the cathodes was progressivelyimproved, however, owing to the reduced impurities of antimony andbismuth in the cathodes.

Subsequent to reaching the steady state, the original anodes, which hadan antimony content of 350 g/t, were replaced with anodes having anantimony content of 800 g/t. The good quality of the cathodes wasunaffected by the high antimony content of the anodes owing to the factthat no floating slime was formed.

As opposed to conventional electrolysis, no deposits were obtained inthe conduit sytems.

Anodes having an antimony content of 1,100 g/t also gave the same goodresult. The total antimony content stabilized at 0.2 g/l, of whichSb(V)-constituted less than 0.02 g/l and the bismuth content at lessthan 0.1 g/l. Analysis carried out on wirebars manufactured fromcathodes obtained by electrolysing anodes con taining 1100 g/t antimonygave the following results calculated in g/t:

Ag Fe Ni Pb Bi Sb As Te Se S 7 1.5 1.5 0.l 0.3 0.5 (H 0.4 7

Analyses made on wirebars manufactured from cathodes obtained from aconventional electrolyses process with only 350 g/t antimony in anodesgave the following result:

Se S 0.4 7

Ag Fe Ni 10 7 1.5

What I claim is:

1. A method for electrolytically producing cathode copper by theelectrolysis of copper anodes containing at least one of the substancesantimony and bismuth in quantities exceeding 400 grams per ton withrespect to antimony and 200 grams per ton with respect to bis muth,characterized by supplying arsenic ions to the electrolyte in suchquantities that in the steadystate the content of trivalent arsenicexceeds 1 g/l and the content of pentavalent arsenic exceeds 2 g/l, andby ensuring that the content of pentavalent antimony in the steady statedoes not exceed 0.05 g/l.

2. A method according to claim 1, characterized by adding trivalentarsenic to the electrolyte in the form of arsenic trioxide.

3. A method according to claim 1, characterized by adding pentavalentarsenic to the electrolyte in the form of arsenic pentoxide.

4. A method according to claim 3, characterized in that the arsenicpentoxide is added to the electrolyte in the form of an aqueoussolution.

5. A method according to claim 1, characterized in that trivalentarsenic is added to the electrolyte by means of arsenic alloyed in theanodes.

6. A method according to claim 1, characterized by withdrawingelectrolyte from the main circulating flow thereof, supplying arsenicions to the withdrawn electrolyte and recirculating said electrolytewith the main flow thereof.

2. A method according to claim 1, characterized by adding trivalentarsenic to the electrolyte in the form of arsenic trioxide.
 3. A methodaccording to claim 1, characterized by adding pentavalent arsenic to theelectrolyte in the form of arsenic pentoxide.
 4. A method according toclaim 3, characterized in that the arsenic pentoxide is added to theelectrolyte in the form of an aqueous solution.
 5. A method according toclaim 1, characterized in that trivalent arsenic is added to theelectrolyte by means of arsenic alloyed in the anodes.
 6. A methodaccording to claim 1, characterized by withdrawing electrolyte from themain circulating flow thereof, supplying arsenic ions to the withdrawnelectrolyte and recirculating said electrolyte with the main flowthereof.